The present invention relates to a process for producing a composite resin for use in a toner for developing an electrostatic image in electrophotography.
A heating roller fixation method wherein a toner image formed on an image receiving sheet is permanently fixed on the sheet by thermal fusion is popular in the electrophotography. In this method, since the surface of a heating roller is press-bonded to the surface of an image on a fixation sheet, the heat efficiency in heat fusion of the toner image on the fixation sheet is so excellent that rapid fixation can be conducted, which renders this method very favorable for use in an electrophotographic copying machine.
On the other hand, in the above-described heating roller fixation method, the toner should preferably satisfy important property requirements such as softening at a relatively low temperature for the purpose of exactly fusing the toner on a fixation sheet (that is, excellent fixation), free from adhesion and retention of the toner on the roller even when the heating roller temperature is relatively high (that is, excellent non-hot-offset property), and the like. A process which is recognized in the art as preferable for satisfying the above-described property requirements comprises homogeneously mixing a low-molecular weight polymer having an excellent fixation with a high-molecular weight polymer having an excellent non-hot-offset property and using the polymer mixture as a resin for a toner. This is disclosed in, for example, Japanese Patent Publication No. 6895/1980, Japanese Patent Laid-Open Nos. 114245/1979 and 36558/1983.
For this reason, various proposals have hitherto been made on methods of homogeneously mixing the low-molecular weight polymer with the high-molecular weight polymer. All these conventional processes, however, have the following problems, and the alleviation in the problems have been desired in the art.
(1) Process wherein a low-molecular weight polymer and a high-molecular weight polymer are separately prepared and both the polymers are mixed with each other in the state of a dry powder. The low-molecular weight polymer is generally different from the high-molecular weight polymer in the softening point and melt viscosity. For this reason, in the case of the mixed polymer produced by this process as a resin for a toner, the low-molecular weight polymer powder is preferentially melt in the step of heat-melt kneading in the production of a toner, and the high-molecular weight polymer powder which has not been sufficiently melt is agglomerated and dispersed in the melt, so that it is unfavorably difficult to homogeneously mix both the polymers with each other. As a result, in the finally obtained toner, scattering of molecular weight distribution occurs between toner particles, which gives rise to problems including that it is difficult to homogeneously mix a compounding agent for a toner, such as a colorant, and no sufficient hot-offset property can be attained.
(2) Process which comprises dissolving a low-molecular weight polymer and a high-molecular weight polymer respectively in different solvents, mixing both the polymers in a solution state and removing the solvent by drying under reduced pressure or spray drying to give a mixed polymer. In this process, when the difference in the molecular weight between both the polymers is large, there occurs a remarkable difference in the solubility of both the polymers in a solvent. This makes it difficult to prepare particularly a highly concentrated solution of the high-molecular weight polymer, so that it becomes difficult to prepare a mixed polymer in any combination of the molecular weights at any mixing ratio. Further, the step of dissolving a polymer, a step of removing a solvent, and the like becomes necessary, which unfavorably increases the production cost.
(3) Process which comprises mixing a low-molecular weight polymer and a high-molecular weight polymer with each other in the form of separate emulsions and subjecting the mixture to steps such as coagulation, washing, dehydration and drying to produce a mixed polymer. In this process, since both the polymers are mixed with each other in the form of a particle having a diameter of 1 .mu.m, the mixture becomes so homogeneous that good toner properties can be obtained. However, a remarkable lowering in the fluidity of the mixed emulsion occurs in the coagulation treatment in the production of the mixed polymer. In order to improve the fluidity, it is necessary to use water in an amount 15 to 20 times that of the polymer. Further, since the resultant mixed polymer particle is in the form of a granule which comprises a loosely solidified emulsion agglomerate having a particle diameter of 5 to 20 .mu.m, the drying property is poor and the mechanical strength is unsatisfactory, which renders this process unsuitable for use on a commercial scale.
(4) Process disclosed in Japanese Patent Publication No. 6895/1980, Japanese Patent Laid-Open Nos. 114245/1979 and 36558/1983, which comprises producing a low-molecular weight polymer by suspension polymerization, subsequently conducting suspension polymerization to produce a high-molecular weight polymer (or a process which comprises previously producing a high-molecular weight polymer by suspension polymerization and subsequently producing a low-molecular weight polymer). In this process, a resin for a toner can be advantageously prepared in a homogeneously mixed state because the polymer particle formed in the first suspension polymerization is dissolved in a monomer added in the second-stage polymerization to continue the suspension polymerization of the monomer. Since, however, it takes a lot of time to prepare a high-molecular weight polymer by suspension polymerization, the production cost becomes high. Further, since the particle diameter of the polymer formed in the first suspension polymerization is relatively large, it takes a lot of time to dissolve the polymer particle in a monomer added at the time of the second-stage suspension polymerization. In some cases, dissolution becomes insufficient, which has an adverse effect on the electrophotographic properties of the finally obtained toner. Further, since there is a limitation on the molecular weight of the high-molecular weight polymer which can be produced by the suspension polymerization, it is difficult to mix a polymer having a high molecular weight sufficient to attain a good offset property.
(5) Process disclosed in Japanese Patent Laid-Open No. 88003/1985 filed by the present applicant which comprises producing a low-molecular weight polymer by suspension polymerization, wherein suspension polymerization is conducted in the presence of a high-molecular weight polymer emulsion particle in the suspension polymerization system to composite the low-molecular weight polymer with the high-molecular weight polymer. In this method, the step of preparing a high-molecular weight polymer through suspension polymerization can be omitted as opposed to the suspension polymerization-suspension polymerization in the above-described process (4). Since the high-molecular weight polymer is in the form of a small emulsion particle, it is easily dissolved in the monomer during the suspension polymerization. This process has features including that any high-molecular weight polymer having a capability of sufficiently imparting a good non-offset property can be mixed in any ratio. In this process, however, a special dispersion stabilizer is necessary for conducting a suspension polymerization with an emulsion system different from the suspension polymerization system being present in the suspension polymerization system. The special dispersion stabilizer has an adverse effect on the environmental stability of the resultant toner.